The object of our studies is to find the molecular determinants in the flow of genetic information that underlie embryonic development and cellular differentiation. Our present effort has been directed at elucidating the origin and fate of messenger RNA, (mRNA) and particularly how the regulation of its synthesis and its stability might change during embryonic development. We are attempting to fractionate heterogeneous nuclear RNA (HnRNA) into fractions that serve as precursor and do not serve as precursor to mRNA. This effort appears to be promising particularly at the early 200-cell blastula stage when histone mRNA's are synthesized in very great abundance. We shall examine the nonpolyadenylated HnRNA for homology to histone mRNA. At later stages we shall examine whether or not polyadenylate containing HnRNA is the exclusive respository of prospective mRNA sequences containing polyadenylate. We have noted changes in the fraction of polyadenylate-containing HnRNA during development from early to late blastula stages. We shall examine animalized and vegetalized embryos which suffer abnormaliteis during these blastula stages, in order to determine whether or not the changes in HnRNA properties reflect crucial changes in regulatory mechanisms needed for normal development and differentiation.